This invention relates to a method and apparatus for controlling the speed of an alternating current (AC) motor with a frequency changer.
The term AC motor utilized herein includes a synchronous motor and an induction motor, and as a frequency changer may be used a current type inverter, a voltage type inverter, a cycloconverter or the like.
FIG. 1 shows an example of a prior art control apparatus of an AC motor which comprises an AC source 11, a rectifier 12, a DC reactor 13, an inverter 14, an induction motor 15, a speed reference setter 16 comprising a variable resistor which sets a desired speed reference an input limiter 17, a frequency controller 18, an output voltage controller 19, an output voltage detector 20, an input current detector 21 in the form of a current transformer, and an input voltage detector 22 which are connected as shown. The control apparatus shown in FIG. 1 is used to control the speed of the induction motor 15 by using a current type inverter in which the AC input from the source 11 is rectified by the rectifier 12, the output DC thereof being smoothed by the DC reactor 13 and then inverted by the inverter 14 into an AC power supplied to the induction motor 15. The AC power supplied to the induction motor 15 is controlled by an output voltage frequency reference signal e.sub.2 obtained by comparing a reference signal e.sub.1 set by the speed reference setter 16 with an input voltage signal in the input limiter 17. The primary frequency of the voltage supplied to the induction motor 15 is determined by varying a gate signal supplied to thyristors constituting the inverter 14 from the frequency controller 18 according to the output voltage frequency reference signal e.sub.2. On the other hand, the primary voltage supplied to the induction motor 15 is controlled by controlling the gate signals applied to the thyristors constituting the rectifier 12 from the output voltage controller 19 according to the output voltage frequency reference signal e.sub.2. Thus, the primary voltage detected by the output voltage detector 20 is compared with the output voltage frequency reference signal e.sub.2 with a comparator 23 in a closed voltage controlling loop, while current is controlled by a minor closed current controlling loop including input current detector 21 so that the output current of the rectifier 12 would not exceed a predetermined value. More particularly, when the output current of the rectifier is caused to increase by a short circuit or an overload of the motor 15, the output current is limited to be less than the predetermined value regardless of the output voltage frequency reference signal e.sub.2 by decreasing the output voltage of the rectifier 12. In other words, within a predetermined output current, the primary voltage and the primary frequency of the induction motor 15 are controlled in a predetermined manner according to the output voltage frequency signal e.sub.2, whereas when the output current tends to increase beyond the predetermined value, the primary voltage is decreased to limit the output current.
The variation in the output voltage of the AC source 11 is detected by the input voltage detector 22. Thus, when the operation of the source 11 is stopped momentarily, the voltage detector 22 momentarily stops the operation of the frequency converter. Such operation mode is illustrated in FIG. 2, in which curve (a) shows the voltage of the AC source 11, (b) a running signal, (c) the number of revolutions of the induction motor 15 and (d) the output voltage frequency reference signal e.sub.2. Suppose now that the voltage of the source 11 decreases below the predetermined value at a time t.sub.1 and that the voltage resumes the predetermined value at a time t.sub.2, then during an interval Tc between t.sub.1 and t.sub.2, the operations of the rectifier 12 and the inverter 14 would be interrupted, and during this interval the number of revolutions of the induction motor 15 decreases by .DELTA.N. For this reason, the output voltage frequency reference signal e.sub.2 is also decreased by .DELTA.E during this interval to a value corresponding to the number of revolutions of the induction motor. This is caused by the torque T-primary current characteristic I of the motor 15 shown in FIG. 3, in which the abscissa represents the number of revolutions N and the ordinate the torque T and the primary current I of the induction motor. When the primary frequency of the induction motor 15 is F.sub.1, the torque is designated by T.sub.1, the primary current by I.sub.1, and the synchronous speed by N.sub.01, whereas when the primary frequency is F.sub.2, the torque is designated by T.sub.2, the primary current by I.sub.2, and the synchronous speed by N.sub.02, where F.sub.1 &lt;F.sub.2. When the primary current is limited to I.sub.21 for the primary frequency F.sub.2 the torque T is the same as T.sub.2 between a speed range of N.sub.21 -N.sub.22, but in other speed ranges the generated torque decreases by an amount shown by hatched portions.
Suppose now that the motor 15 running with the primary frequency F.sub.2 up to time t.sub.1, stops running during the interval Tc and restarts at time t.sub.2 with the primary frequency F.sub.2. Under these conditions, where the speed of the motor 15 is higher than N.sub.21 immediately prior to time t.sub.2, it is easy to return the motor speed to that prior to time t.sub.1, whereas when the speed has been reduced to a value less than N.sub.21 the output current of the inverter 14 (i.e., rectifier 12) would be limited to I.sub.21 with the result that the torque T of the motor decreases, thus making it impossible to resume the number of revolutions before time t.sub.1. Accordingly, when the primary frequency is decreased to F.sub.1 from F.sub.2 corresponding to the decrement .DELTA.N in the speed, it would be possible to make the torque to be T.sub.1 at time t.sub.2 to begin to accelerate the motor at time t.sub.2 to resume the original speed provided that the synchronous speed satisfies a relation N.sub.02 -N.sub.01 .gtoreq..DELTA.N.
From the foregoing description, it will be noted that where the source voltage momentarily disappears, the motor can restart to resume the original speed, but unless the output frequency reference signal e.sub.2 is also decreased by .DELTA.E corresponding to the variation .DELTA.N in the motor speed during the interval Tc, the output current of the inverter would be limited to I.sub.21 which decreases the torque T making it difficult to resume the original speed.
Consequently, how to determine the width of variation .DELTA.N in the speed presents various problems. For example, as the variation width is greatly influenced by the length of the instantaneous interruption interval Tc and the load condition of the induction motor 15, it is necessary to restart the motor by making the decrement .DELTA.E of the output voltage frequency reference signal e.sub.2 to have sufficiently large margin, that is to make small the primary frequency at the time of restarting. For this reason, it takes a long time to resume the normal rotation and if the decrement were too small it would be impossible to resume the normal or original speed.